A conventional belt-driven continuously variable transmission comprises a pair of pulleys, an endless driving belt running between the pulleys, and a hydraulic chamber formed on each pulley. Oil is delivered to the hydraulic chamber of a drive pulley to change a speed ratio, and oil is delivered to the hydraulic chamber of a driven pulley to establish a belt clamping pressure. In the belt-driven continuously variable transmission thus structured, therefore, a speed ratio can be varied continuously by hydraulically changing groove widths of the pulleys to change running diameter positions of the belt.
However, in the belt-driven continuously variable transmission, power transmission is achieved frictionally between the belt and each pulley. That is, if the belt clamping pressure of the pulley is insufficient, a belt slip is caused on a contact face of the pulley. In order to prevent such belt slippage, the belt clamping pressure is controlled depending on a situation of a speed change operation and a transmission torque. To this end, various improvements have been made on a structure of the driven pulley and a hydraulic control system of the driven pulley, as disclosed in PCT international publication WO/2012/127651, and Japanese Patent Laid-Opens Nos. 2009-192018, 11-72151 and 5-203006.
PCT international publication WO/2012/127651 describes a belt-driven continuously variable transmission comprising a hydraulic actuator of a driven pulley, and a torque cam assembly adapted to establish a thrust force by a relative rotation between a pair of cam members. According to the teachings of WO/2012/127651, a piston forming the hydraulic chamber of the driven pulley is integrated with a drive cam member of the torque cam assembly. Japanese Patent Laid-Open No. 5-203006 discloses a transmission in which a hydraulic chamber and a torque sensor are arranged on a back side of a movable sheave of a drive pulley.
Japanese Patent Laid-Open No. 2009-192018 discloses a transmission in which a piston arranged on a back side of a movable sheave forms a hydraulic chamber of a secondary pulley, and in which an axial movement of the piston is stopped by a stopper member. According to the teachings of Japanese Patent Laid-Open No. 2009-192018, a gear for outputting torque of the secondary pulley is interposed between two stopper members in an axial direction, and one of the stoppers is interposed between an output gear and the piston. That is, one of the stopper members is brought into contact to the piston in the axial direction.
Japanese Patent Laid-Open No. 11-72151 describes a transmission in which a hydraulic chamber is formed by a movable cylinder attached to a movable sheave to serve as a hydraulic servo mechanism, and a fixed plunger fixed to a pulley shaft by the stopper member.
According to the teachings of WO/2012/127651, however, a bearing supporting the output gear is brought into contact to one of the cam member in the axial direction and hence the bearing is subjected to an axial load through contact faces of those members. For this reason, the bearing may be damaged by a thrust load. In addition, since the piston serving as the cam member is reciprocated hydraulically, a thrust load derived from hydraulic pressure is also applied to the bearing through the torque cam assembly.
According to the teachings of Japanese Patent Laid-Opens Nos. 2009-192018 and 11-72151, the stopper member for restricting an axial movement of the piston is disposed between the piston and the output gear. According to the teachings of Japanese Patent Laid-Open No. 5-203006, axial movement of one of the cam members can be restricted. However, according to the teachings of Japanese Patent Laid-Opens Nos. 2009-192018 and 11-72151, an actuating device for moving the piston toward the back side of the movable sheave is not available in the transmission. Likewise, according to the teachings of No. 5-203006, the torque sensor is not adapted to establish a thrust force for moving the movable sheave. According to the teachings of those prior art documents, therefore, belt slippage may be caused when hydraulic pressure applied to the hydraulic chamber is lowered by a trouble.